The present invention relates to molecular monolayers compositions and methods of forming the same.
The interest in two-dimensional (2D) materials results from the fact that optoelectronics and molecular electronics have become frontier areas of material science (Ulman, 1991). Multilayered organic structures have recently received theoretical (Lam et al, 1991) and experimental (So et al, 1991; Forrest et al, 1994; Haskal et al, 1994; Ohmori et al, 1993; Yoshimura et al, 1991; Yoshimura et al 1992; Donovan et al, 1994; Donovan et al, 1993) treatment. Novel and applicable photophysical properties of organic superlattices have been predicted, including enhancement of optical nonlinearities (Lam et al, 1991; Zakhidov and Yoshino, 1994) and photoelectric transformations (Zakhidov and Yoshino, 1994). Techniques such as organic molecular beam deposition (OMBD) (So et al, 1991; Forrest et al, 1994; Haskal et al, 1994; Ohmori et al, 1993) have already proved the capability of growing ultrathin layers having the quality of inorganic quantum well (QW) structures.
A number of interesting optical and photophysical phenomena have already been found in OMBD derived organic QW, including the observation of exciton confinement in photoluminescence (PL) (So et al, 1991; Forrest et al, 1994) and electroluminescence (EL) and electric field modulation of PL (Ohmori et al, 1993). Preparation of crystalline thin organic films by the OMBD relies on the bonding of molecular layers via weak van der Waals forces to achieve and preserve quasi-epitaxial structures (Forrest et al, 1994). Thus, perfect monolayers without step edges cannot be achieved and the lower limit is an average of three xe2x80x9cmonomolecularxe2x80x9d, layers. A solution to these limitations can be found in another ultrahigh vacuum (UHV) technique: molecular layer deposition (MLD) (Yoshimura et al, 1991; Yoshimura et al, 1992). MLD demonstrated the construction of quantum dots and quantum wires and the potential use of functionalized organic precursors to form alternating multilayered structures. This approach is similar to (inorganic) atomic layer deposition (ALD) (Pessa et al, 1981) and the solution analog-molecular self-assembly (MSA) (Ulman, 1991).
A solution derived method to produce 2D layered structures, the Langmuir-Blodgett (LB) technique, yields films exhibiting anisotropic electron transport (Donovan et al, 1994) and tunneling (Donovan et al, 1993), again suggesting QW behavior. While the LB method is useful in achieving 2D multilayered physiadsorbed structures, LB films suffer from low chemical and thermal stability and cannot incorporate large chromophores without phase-segregation and micro-crystal formation. Alternatively, the trichlorosilane-based MSA approach 1 provides the advantages of strong chemiadsorption through Sixe2x80x94O bonds, chemical and thermal stability, and the ability to form non-centrosymmetric structures (Yitzchaik et al, 1993; Li et al, 1990).
Vapor phase growth techniques (Kubono et al, 1994) such as vapor deposition polymerization (VDP) of thin films was recently demonstrated for aromatic polymers such as polyimides (Maruo et al, 1993), polyamides (Takahashi et al, 1991), polyureas (Wang et al, 1933), and polyether-amines (Tatsuura et al, 1992). In the VDP process, two types of monomers are evaporated onto the surface of a substrate in a vacuum chamber. Condensation or addition polymerization then takes place between deposited monomers to produce thin polymeric films. Thin polymer films of high quality and uniformity can be fabricated by this process (Maruo et al, 1993; Takahashi et al, 1991). Thermally stable piezo- and pyro-electrical properties were found in poled samples (Wang et al, 1993). Moreover, electric field assisted VDP (in situ poling of hyperpolarizable monomers) was employed to fabricate electro-optic polymer waveguides (Tatsuura et al, 1992).
In one aspect, the invention includes a method of forming a multilayered structure composed of two or more discrete monomolecular layers, where at least one layer is composed of molecules of a selected polycyclic aromatic compound having a defined axis oriented substantially upright with respect to the plane of the monolayer, e.g., normal to the plane, or within up to 54xc2x0 of normal. The said axis is typically the molecule""s z axis, namely, the longest axis of the molecule. The method includes depositing molecules of a selected aromatic compound, preferably a polycyclic aromatic compound, having a defined axis with a chemically reactive group at each axial end, by vapor phase deposition, onto a substrate having surface-reactive sites capable of reacting with the chemically reactive group in the selected compound. The deposition step is carried under conditions which allow chemiadsorption of the selected compound in a molecular monolayer, by covalent coupling of one end of the compound to the substrate, and sublimation of non-covalently bonded compounds from the surface. There is formed by the deposition step a monomolecular layer of the selected compound. For some applications of the multilayered structures of the invention it may be preferable that the monolayer formed by the said deposition is characterized by in-plane compound ordering. These steps are carried out one or more times, where the monomolecular layer formed at each deposition cycle forms a new substrate having a surface-exposed monolayer with exposed reactive groups.
In one general embodiment, the method includes reacting the surface-exposed monolayer with a bifunctional reagent that reacts with the exposed reactive groups forming the just-deposited layer, to produce a coupling layer having exposed reactive groups with which the reactive groups of the selected compound forming the next monolayer can react.
For example, the surface-reactive groups on the substrate may be amine groups, and the bifunctional reagent may be a diamine compound. In this embodiment, the selected compound may be, for example, a polycyclic tetracarboxylic-dianhydride compound, capable of forming axial-end imide linkages, a polycyclic diacyl halide, capable of forming axial-end amide linkages, a polycyclic dialdehyde, capable of forming axial-end Schiff base linkages, and a polycyclic diisocyanate, capable of forming axial-end urea linkages.
The deposition in such an embodiment may be carried out, for example, N times in succession with one selected polycyclic compound, and M times in succession for a second selected polycyclic compound, to form a N monolayers of the one compound, and M monolayers of the other compound. The first and second polycyclic may be, for example, perylene and naphthalene compounds, respectively.
Alternatively, in the embodiment, the deposition steps may be carried out N times in succession for a first selected polycyclic compound, and one or more times in succession for the bifunctional reagent.
In another general embodiment of the method, the surface-reactive groups on the substrate are maleimide groups, the selected compound is a polycyclic compound with z-axis amine groups, such as a diaminocarbozole, and the bifunctional reagent is a bismaleimide compound.
The method is useful for example, in forming organic monolayers in an organic light-emitting diode (OLED), and OLED array, organic field effect transistor (OFET), non-linear optical devices, photoreceptors and solar cells, waveguides, and a supercapacitor.
In another general embodiment, the invention includes a polymer-based layered-structure comprising (i) a substrate, (ii) a first monomolecular layer composed of monomers of a selected aromatic compound, preferably a polycyclic compound, having a defined z axis oriented substantially upright with respect to the plane of the monolayer, e.g., normal to the plane, or within about 0-54xc2x0 of normal, with the monomers forming the monolayer being covalently attached at one axial end to the substrate, and (ii) a second monomolecular layer composed of monomers of a selected polycyclic aromatic compound having a defined z axis oriented substantially normal to the plane of the monolayer, with the monomers forming the monolayer being covalently attached at one axial end to axial end of molecules forming the first monolayer. Exemplary monolayer compositions are as above.
The compositions find applications, for example, in organic monolayers in an organic light-emitting diode (OLED), an OLED array, organic field effect transistor (OFET), electronic-switch, non-linear optical devices, photoreceptors, waveguides, and supercapacitors.